Intermittent Overload Trip on a Conveyor Gearmotor
A 7.5 kW, 400 V, 50 Hz conveyor gearmotor drives a bulk bag unloader for 10 hours per day. The motor nameplate full-load current is 15.2 A at 400 V. After about 25 minutes of steady operation the thermal overload relay trips; resetting restores running for another 20–30 minutes. The relay is set to 15.5 A and is mounted in a cabinet whose internal temperature reaches 46 °C in summer; the cabinet fan has failed. A clamp meter shows phase currents of 15.9 A, 16.2 A and 15.1 A. Line-to-line voltages are 402 V, 391 V and 398 V. A vibration reading at the gearbox input bearing is 4.6 mm/s RMS overall, with a visible peak at 1× running speed; the motor shaft alignment was last checked two years ago. The conveyor is mechanically free when turned by hand, and the gearbox oil level is correct. Production wants a quick reset and continued running.
1. Calculate the voltage imbalance and assess whether the motor is operating within its thermal capability. Show your reasoning.
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Average voltage = (402 + 391 + 398) / 3 = 397 V. The greatest deviation from the average is 6 V, so imbalance = 6 / 397 × 100 = 1.51%. For a polyphase induction motor, even about 1.5% voltage imbalance causes a disproportionate negative-sequence current and extra rotor heating; a reasonable derating is roughly 96–97% of nameplate current. That gives an allowable current near 14.6–14.8 A, not 15.2 A. The measured 16.2 A is about 107% of nameplate and well above the derated value. The 46 °C cabinet temperature further reduces the motor's thermal margin because the motor is rated on a 40 °C ambient basis. The relay set at 15.5 A may be ambient-compensated, but that does not restore the motor's lost cooling. Conclusion: the motor is thermally overloaded by the combination of supply imbalance, high ambient and mechanical load; raising the relay setting would be unsafe.
2. List the likely causes in priority order and describe a diagnostic sequence that avoids unnecessary motor replacement.
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First verify the electrical supply: measure all three line-to-line voltages at the motor terminals under load, check for loose lugs, pitted contactor tips, single-phasing and whether large single-phase loads share the distribution. Next check the mechanical train: shaft alignment, belt or chain tension, gearbox input bearing condition and conveyor drag under actual load. The 1× vibration peak supports misalignment or unbalance, both of which raise current. Then test the motor: winding resistance balance, insulation resistance and no-load current. Finally check the overload relay: correct heater range, trip class, ambient compensation and cabinet cooling. Use a clamp logger to capture current and voltage over a full cycle, thermal imaging to compare phase and bearing temperatures, and a vibration spectrum to confirm alignment or bearing faults. Do not replace the motor until supply and mechanical causes have been eliminated, because a new motor in the same conditions will trip again.
3. State the immediate action you would take and what records you would require before returning to full production.
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Stop repeated resetting. Isolate the drive, restore cabinet cooling, and allow a controlled trial only at reduced load or with a temporary run-time limit. Correct the voltage imbalance and check alignment before returning to full production. Record phase currents and voltages, cabinet and motor temperatures, trip times, relay range and trip class, vibration spectrum, alignment readings and gearbox condition. After correction, run the conveyor loaded and trend current and temperature for at least one full production cycle. The motor should run below the derated current with stable vibration and no abnormal temperature rise. If the current remains above the derated value after electrical and mechanical corrections, resize the motor or reduce the load rather than increasing the overload setting.